Titanium oxide produces an OH radical having a large energy corresponding to 120 kcal/mol by irradiation of light. This energy is larger than the bond energy of a C--C bond, a C--H bond, a C--N bond, a C--O bond, an O--H bond, or an N--H bond of organic compounds and thus the energy can easily decompose these bonds. Because of this action, titanium oxide can easily decompose organic compounds, and therefore it has been used as a catalyst of photodecomposition reactions to decompose a variety of organic substances including the decomposition, sterilization etc. of harmful substances or bad-smelling substances dissolved in water or suspended in the air, thereby finding practical applications in environmental clarification, epidemics control, and the like.
Such a utilization of titanium oxides as a catalyst can be effectively carried out by forming titanium oxide thin films on the surface of a substrate including ceramics such as glass and tiles or inorganic fibers. On the other hand, by utilizing properties that the titanium oxide thin films have high refractive indices and are chemically stable, titanium oxide thin films formed on the surface of light-transmitting substances such as glass have been used for optical applications such as coating of optical lenses and for heat radiation reflexive glasses, and are expected to be a solar cell which makes possible low-cost solar power generation.
As the processes for forming a titanium oxide thin film on the surface of a substrate, there are mentioned CVD, ion plating, sputtering, and the like. In these processes, however, special and expensive equipment is required, and the formation of a thin film on the substrate having a large surface area and the formation of a homogeneous thin film on the surface of a substrate having a complicated shape are difficult.
Furthermore, there are a process for forming titanium oxide thin films by spraying onto the surface of a substrate a solution in which a titanium alkoxide such as tetraethyl titanate, tetra-n-propyl titanate, tetraisopropyl titanate, or tetra-n-butyl titanate; or a titanium chelate compound such as titanium acetyl acetonate is dissolved in an organic solvent, or by soaking a substrate in said solution, removing the solvent and then by oxidizing at a high temperature, or a process for exposing the substrate to the vapor of titanium tetrachloride. However, in addition to being complicated, these processes require dealing with hydrolytic substances and so it is impossible to form a uniform thin film on the substrate having a large surface area or the substrate having a complicated shape.
Another process for forming titanium oxide thin films on the surface of a substrate involves spreading a mixture obtained by kneading finely powdered titanium oxide with a binder and a dispersant on the surface of a substrate followed by drying thereof. However, in such a process, it is difficult to form a sufficiently thin and uniform as well as a strong thin film. Furthermore, due to the photocatalytic activity possessed by titanium oxide, the binder which is an organic substance is decomposed, and therefore adhesiveness onto the substrate cannot permanently continue.
In Japanese Unexamined Patent Publication No. 59-141441 and Japanese Unexamined Patent Publication No. 1-93443, processes for forming a titanium oxide-coated layer on the surface of the substrate by soaking a substrate in an aqueous solution containing fluorotitanate in the presence of a fluoride ion-capturing agent such as boric acid have been disclosed. In the former, zinc oxide is saturated in said aqueous solution so that there is a problem of causing variation in refractive index due to introduction of Zn atom into the coated layer. The concentration of fluorotitanic acid is 0.3 to 1.0 mol/L in the former and 0.1 to 3 mol/L in the latter, and a fluorotitanic acid solution of 0.5 to 3.4 mol/L has been used in the Example therein. When such a high concentration of fluorotitanic acid is used, the solution becomes turbid with the addition of a fluoride ion-capturing agent since a large quantity of titanium oxide precipitates in the solution, which prevents formation of a thin film on the surface of the substrate without wasting the materials. Furthermore, the formed thin film is ununiform and the surface condition thereof is bad so that it is not suitable for optical applications in which its transmittance to light and its uniformity are required and for photoreactive catalysts or solar cells.
Thus, an object of the present invention is to provide a process for forming a titanium oxide thin film excellent in uniformity on the surface of a substrate by means of simple equipment by overcoming the abovementioned problems associated with the conventional processes. Another object of the present invention is to provide a catalyst for photodecomposition reaction, said catalyst comprising a titanium oxide thin film formed on the surface of the substrate.